1. Field of the Invention
The present invention relates to improvement in a zoom lens used as a photography optical system for a variety of cameras including a so-called silver camera, and more particularly, to a zoom lens suitable for cameras such as a digital camera and a video camera, and a camera and a portable information terminal device using such a zoom lens.
2. Description of the Prior Art
In recent years, instead of a conventional camera using a silver film, i.e., a sliver camera, there has been rapidly known a camera of such type referred to as a digital camera, an electronic camera or the like, which photographs an image of a subject (object) by a solid-state imaging device such as a CCD (charge coupled device), thereby acquiring image data such as a static image (still image) or a motion image (movie image) of the object and digitally recording the data in a nonvolatile semiconductor memory represented by a flash memory.
A market for such a digital camera is very large, and the user demands for the digital camera have been ranged in scope. In particular, high image quality and miniaturization of a digital camera are always the user's demand, and are of large interest of the user. Therefore, compatibility of high performance and miniaturization is requested for a zoom lens used as a photographing lens.
On the other hand, it is desirable that a photographing lens mounted on a camera is as bright as possible, i.e., capable of capturing a large amount of light. In particular, in the case of a zoom lens, a telephoto end is prone to darker than a wide angle end, and it is desired to ensure brightness at the telephoto end. This is because of the absence of sufficient brightness at the telephoto end delays a shutter speed at the telephoto end, and causes a motion of hands or a motion of an object.
On the other hand, if such insufficient brightness is compensated for by an increase of gain of an imaging device, it causes higher noise.
In addition, there is another problem that, if there is a significant change of brightness in accordance with zooming from a wide-angle end to a telephoto end, exposure control during monitoring using a display element becomes complicated. For example, in the case where zooming from the wide angle to the telephoto end is carried out while in monitoring, if there is no change of brightness of a lens in accordance with zooming, there is no unnaturalness even if exposure control is not carried out while in zooming.
However, in the case where a lens is darkened at the telephoto end, if no exposure control is carried out while in zooming, a monitor image is gradually darkened. Thus, it is desirable to provide a zoom lens in which there is a small change of brightness in accordance with zooming from the wide-angle end to the telephoto end.
There are various types of zoom lenses used for a photographing lens of a digital camera. A zoom lens of such type suitable for miniaturization is characterized by including: a first group optical system having a negative focal length, a second group optical system having a positive focal length, and a third group optical system having a positive focal length, which are sequentially arranged in this order from an object side; and a aperture stop which is disposed on the object side of the second group optical system and moves integrally with the second group optical system, wherein, in change of magnification from a short focal end to a long focal end, the second group optical system monotonously moves from the image side to the object side, and the first group optical system moves so as to correct displacement of an image plane position in accordance with the change of magnification.
A zoom lens of such a type is disclosed in Japanese Patent Laid-open (JP-L) Nos. 10-39214, 11-287953, and 2001-296476 or the like, for example.
Among them, the above zoom lens of such type disclosed in JP-L No. 10-39214 was filed at the earliest time, and a basic construction of the above zoom lens of such type is all disclosed. In addition, in each of JP-L Nos. 11-287953 and 2001-296476, there is disclosed an improved zoom lens which is an improvement in the previous zoom lens disclosed in JP-A No. 11-287953, and is a miniaturization of the previous zoom lens.
However, all of the zoom lenses disclosed as embodiments or specific examples in the above described JP-L Nos. 10-39214, 11-287953, and 2001-296476 have a disadvantage that they cannot acquire sufficient brightness at a telephoto end, and include a significant change of brightness in accordance with zooming from the wide-angle end to the telephoto end.
For example, in Example 1 of JP-L No. 10-39214, “F/No. (F number)=2.5 to 4.4 when “f(focal length)=4.6 mm to 14.6 mm” is described. Namely, the brightness in a state in which the focal length “f” is the longest, 14.6 mm, namely, the F number is 4.4, which is not sufficiently bright. In addition, a change of F number is in the range of 2.5 to 4.4, and a change of brightness is significant.
Similarly, in Example 2 of JP-A No. 10-39214, “F/No.=2.4 to 4.3” when “f=4.6 mm to 14.0 mm” is described, and in Example 3 thereof, “F/No.=2.5 to 4.8” when “f=4.5 mm to 15.0 mm” is described, respectively. The brightness, in a state in which a telephoto end, namely, a focal length “f” is the longest, 14.0, and 15.0 mm, namely, the F number is 4.3 and 4.8, which is not sufficiently bright. In addition, a change of F number is in the range of 2.4 to 4.3 and in the range of 2.5 to 4.8, a change of brightness is also significant.